This patent relates to catalysts supported on a foraminous carrier and methods for preparing such catalysts using stabilized aqueous compositions. In particular, this patent relates to aqueous compositions containing catalytically-active metal components and substantially water soluble acidic components and to the catalysts prepared using such aqueous compositions for impregnating foraminous carriers. It is desirable to convert heavy hydrocarbon feeds, including those having a boiling point below about 1200° F., into lighter, and more valuable, hydrocarbons. It is also desirable to treat hydrocarbon feedstocks, including petroleum residues, also known as resid feedstocks, in order to carry out, for example, hydrodesulfurization (HDS), hydrodenitrogenation (HDN), carbon residue reduction (CRR), hydrodemetallation (HDM), including the removal of nickel compounds (HDNi) and vanadium compounds (HDV). The catalysts of the present invention are particularly useful and effective in the hydrodesulfurization, hydrodenitrogenation, hydrodemetallation, etc. of petroleum compositions, including high-boiling petroleum compositions.
Catalysts comprising at least one Group VIII metal component, at least one Group VIB metal component and a phosphorus component, such components being carried on a foraminous carrier, are known in the art.
It is known that the metals of Group VIB of the periodic table, for example, tungsten and molybdenum, and components comprising such metals, for example, compounds such as the oxides and sulfides, are active in catalyzing a wide variety of reactions including among others, hydrogenation, dehydrogenation, oxidation, desulfurization, denitrogenation, isomerization and cracking. However, catalytic metals and components containing them are relatively costly and have a relatively small surface area per unit weight, so that they are typically not used without resort to carrier materials. Consequently, these catalytically active metals or components are usually applied in a diluted form to the surface of a foraminous support material. The foraminous support material is usually of a lower order of activity when compared to the catalytically-active components, or such carriers may even be catalytically completely inactive.
Furthermore, it is known that certain metal-containing components of Group VIII of the periodic table of the elements, such as iron, cobalt, and nickel, when used in combination with the Group VIB metal-containing components, result in enhanced catalytic activity. These Group VIII components are sometimes referred to as catalyst “promoters.” However, problems can result when these promoters are attempted to be impregnated into a carrier along with the catalytically active components of Group VIB. Simple and direct impregnation techniques using a mixture of both components typically cannot be employed. For example, a combination of components based on cobalt or nickel salts with molybdenum or tungsten components typically results in unstable solutions, e.g., solutions subject to the formation of precipitates. Impregnation of a carrier using separate solutions comprising components of Group VIB and Group VIII is not an acceptable alternative since that can result in costly, multi-step processes and ineffective or non-uniform metals distribution.
Rather costly and involved processes have been devised in order to obtain a uniform distribution throughout the available surface area of the foraminous catalyst carrier material when using components containing both of the catalytically active metals of Group VIB and Group VIII. It has been the objective of these methods to prepare solutions containing metals of both Group VIB and Group VIII that are sufficiently concentrated and of the requisite stability to allow subsequent uniform impregnation and distribution of the metals throughout and upon the surface area of the carrier. These methods typically include the use of high concentrations of phosphoric acid. Typically, the carrier is impregnated with a dilute solution comprising a phosphorus component, although some applications do not use a phosphorus component, and components of metals of both Group VIB and Group VIII, by applying the solution to a calcined foraminous carrier material, and then drying and calcining the composite to convert the catalytically active material to other forms, particularly to the oxide. However, the use of phosphoric acid, particularly at high concentrations that are required to readily solubilize both of the metal containing components and maintain them in a stable solution, can introduce performance related problems during the use of such catalysts in hydroconversion processes.
Furthermore, there is increased interest in producing very low sulfur and nitrogen crude oil fractions and in producing and upgrading lower quality hydrocarbon feeds, such as synthetic crudes and heavy petroleum crude oil fractions. Unfortunately, high concentrations of nitrogen, sulfur, metals and/or high boiling components, for example, asphaltenes and resins, in such lower quality feeds render the same poorly suited for conversion to useful products in conventional petroleum refining operations. In view of such difficulties, lower quality hydrocarbon feeds often are catalytically hydrotreated to obtain materials having greater utility in conventional downstream refining operations. Catalytic hydrotreating or hydroconversion involves contacting such a feed with hydrogen at elevated temperature and pressure in the presence of suitable catalysts. As a result of such processing, sulfur and nitrogen in the feed are converted largely to hydrogen sulfide and ammonia which are easily removed. Aromatics saturation and cracking of larger molecules can also be used in order to convert high boiling feed components to lower boiling components. Metals content of the feed decreases as a result of deposition of metals on the hydrotreating catalyst.
As can be appreciated, satisfactory operation in processing feeds containing high levels of impurities under severe process conditions places increased demands on the catalyst to be employed as the same must exhibit not only high activity in the presence of impurities and under severe conditions, but also stability and high activity maintenance during the time that it is in use. Catalysts containing a Group VIB metal component, such as a molybdenum and/or tungsten component, promoted by a nickel and/or cobalt component and supported on a porous refractory inorganic oxide, are well known and widely used in conventional hydrotreating processes; however, the same often are somewhat lacking in stability and activity maintenance under severe conditions.
It is known that preparation of hydrotreating catalysts containing Group VIB and Group VIII metal components supported on a porous refractory inorganic oxide can be improved through the use of phosphoric acid impregnating solutions of precursors to the Group VIB and Group VIII metal components or the use of phosphoric acid as an impregnation aid for the metal precursors. Thus, Pessimisis, U.S. Pat. No. 3,232,887 discloses stabilization of Group VIB and Group VIII metal-containing solutions through the use of water-soluble acids. According to the patentee, in column 3, lines 6-11, “in its broadest aspect the invention comprises the preparation of stabilized aqueous solutions which comprise an aqueous solvent having dissolved therein catalytically active compounds containing at least one element from Group VIB of the periodic table and one element from Group VIII.” Inorganic oxyacids of phosphorus are included among the disclosed stabilizers, and the examples of Pessimisis illustrate preparation of various cobalt-molybdenum, nickel-molybdenum, and nickel-tungsten catalysts using phosphorus and other acids as stabilizers. Hydrodesulfurization results with certain of the cobalt-molybdenum catalysts are presented, and the patentee suggests that the use of the stabilized solutions may lead to improved hydrodesulfurization activity in some instances.
Other patents relating to hydroconversion or hydrotreating processes disclose various catalysts, their method of preparation as well as their use in such processes. For example, Simpson et al., U.S. Pat. No. 4,500,424 and its divisional patent, U.S. Pat. No. 4,818,743 are directed to hydrocarbon conversion catalysts containing at least one Group VIB metal component, at least one Group VIII metal component, and a phosphorus component on a porous refractory oxide having a defined and narrow pore size distribution. The catalyst is said to be useful for promoting various hydrocarbon conversion reactions, particularly hydrodesulfurization. Similarly, Nelson et al., U.S. Pat. No. 5,545,602 is directed to hydrotreatment of heavy hydrocarbons to increase content of components boiling below 1000° F. by contact with Group VIII non-noble metal oxide and Group VIB metal oxide on alumina having specific and defined surface area and pore size distribution. This patent also teaches, at column 9, lines 36-37, to avoid adding phosphorus-containing components during catalyst preparation because “Presence of phosphorus undesirably contributes to sediment formation.” In furtherance of this teaching it is suggested, at lines 54-57, that impregnating solutions may be stabilized with H2O2 so that solutions stabilized with H3PO4 not be used. See also Dai et al., U.S. Pat. Nos. 5,397,956 and 5,498,586 similarly directed to defined carrier properties for improved hydroconversion catalysts.
In the catalysts of the present invention, the metals are typically activated by converting them into the corresponding metal sulfides. This can be accomplished by introducing the catalyst comprising the impregnated metals in a device or reactor and conducting a suitable pre-sulfurization treatment, wherein the catalyst layer is sulfurized by introducing a hydrocarbon oil containing an appropriate sulfurizing agent at elevated temperature. The active site of the thus-pretreated catalyst is formed on the surfaces of the resulting active metal sulfides so that the total number of active sites increases with an increase in the exposed surface area of the active metal sulfides, yielding a high catalyst activity. An increase in exposed surface area of the active metal sulfides may be attained by enhancing dispersion of the active metal sulfides as carried on the catalyst carrier or by minimizing the crystal size of the respective active metal sulfides. Methods for preparing such a catalyst include dipping a catalyst carrier in an aqueous solution of active metals containing a carboxylic acid, such as citric acid or malic acid, as a complexing or chelating agent for active metals and thereafter firing the impregnated carrier. For example, EP 0181035(A2) discloses a method of preparing a catalyst in which an organic compound having a nitrogen-containing ligand (e.g., amino group, cyano group) such as nitriloacetic acid, ethylenediaminetetraacetic acid or diethylenetriamine is used as a complexing agent and is added to an aqueous solution of active metals, a catalyst carrier such as an alumina or silica is dipped in the resulting aqueous solution of active metals, and the catalyst composed of active metals carried on the catalyst carrier is then dried at a temperature of not higher than 200° C. without firing. Similar methods are disclosed in U.S. Pat. No. 5,200,381 wherein the complexing agent is a hydroxycarboxylic acid, for example citric acid, and the agent is used in solution with the active metals and the resulting impregnated carrier is kneaded and shaped; or in U.S. Pat. No. 5,232,888 the agent is added to a catalyst already containing the active metals deposited on a carrier. In both of these patents the resulting composition is heated and dried at a temperature not higher than 200° C., and in the absence of calcination, in order to avoid decomposition of the complexing agent.
In accordance with the method of adding a hydroxycarboxylic acid as a complexing agent followed by firing, the acid can be effective for increasing the stability of the active metal-dipping solution as it acts as a complexing agent for active metals and additionally the acid is also effective for inhibiting coagulation of active metals. However, the active metals are subject to coagulation because of the final firing step thereby tending to decrease catalyst activity. On the other hand, in accordance with the method disclosed in EP 0181035(A2), since the active metal ions such as Mo or Ni ions are firmly coordinated with the nitrogen-containing compound, such ions are carried on the carrier in a highly dispersed condition. In addition, since the catalyst containing the active metals is not calcined but is merely dried at a low temperature, not higher than 200° C., and since sulfurization is conducted with unoxidized metals, the active metals can be maintained in a dispersed state. While such catalysts exhibit desirably high activity, they are deficient in that the low temperature heating and drying step does not remove sufficient water, thereby requiring that such water be removed during the initial stages of hydroprocessing, an undesirable and burdensome characteristic. In particular, the water driven off at this early stage in the use of the catalyst can place an undue burden on plant equipment that separates water from the processed petroleum fractions.
Notwithstanding the diverse teachings of patents and publications in respect of the preparation of hydroprocessing or hydrotreating catalysts, there is a continuing need for development of improved catalysts, particularly catalysts that are effective but contain lower levels of moisture as delivered to the petroleum processor.